1. Field of the Invention
This invention relates to a process for producing hydrophobic colloidal silica.
2. Description of the Prior Art
Iler in U.S. Pat. No. 2,786,042, issued Mar. 19, 1957, described a process of adding an aqueous solution of a silanolate which is a strong alkali salt of a monovalent hydrocarbon-substituted silanol to an aqueous sol of colloidal particles of amorphous silica. These treated silica particles could be recovered by drying or they would be transferred from the original aquasol to an organosol by mixing the aquasol after treatment at a pH below 6 with an organic liquid and removing the water.
Another method of producing hydrophobic silicas was shown by Wetzel in U.S. Pat. No. 2,802,850, issued Aug. 13, 1957, wherein a lower alkyltrichlorosilane was hydrolysed in a water-in-oil emulsion containing an alkali-metal silicate. The treated silica was filtered from the oil, washed with water, and dried to yield the desired product. The treated silica is suitable for the preparation of greases and as fillers for elastomeric materials.
Youngs in U.S. Pat. No. 3,634,288, issued Jan. 11, 1972, prepared a hydrophobic silica sol by emulsifying a silica hydrosol in an oil dispersing medium, stripping water from the system, adding a silicone oil, and heating to allow the silicone oil to react with the silica-OH groups, resulting in an oil dispersion of the hydrophobic silica.
These processes yielded a dried, treated silica particle that was hydrophobic, or an organosol of the treated silica. The process of the instant invention provides a storage stable water emulsion of hydrophobic colloidal silica. This emulsion may be dried to yield a crusty, white material which may be easily ground to a fine fluffy white powder. The treated powder is hydrophobic and oleophilic.
Johnson et al. teach in U.S. Pat. No. 4,221,688, issued Sept. 9, 1980, a silicone emulsion which provides an elastomeric product. Their emulsion consists essentially of an anionically stabilized hydroxylated polydiorganosiloxane which contains about 2 silicon-bonded hydroxyls per molecule, an organic tin compound, and a colloidal silica, the emulsion having a pH in the range of 9 to 11.5 inclusive.